1. Field of the Invention
The present invention relates to apparatus and methods for controlling temperature of print heads in a printer apparatus. More particularly, the present invention is directed to a print engine that comprises plural printing stations.
2. Description Relative to the Prior Art
In the prior art as represented by U.S. Pat. No. 5,440,328, thermal printer apparatus are known that operate as a single pass, multi-color thermal printer. In such a printer a print engine is provided that comprises a receiver transport system and three or more thermal print head assemblies. Each of the print head assemblies includes a respective reloadable thermal ribbon cassette which is loaded with a respective color transfer ribbon. Each of the thermal print head assemblies comprises a cantilevered beam, a mounting assembly and a thermal print head having a thermal print line. Each of the print head assemblies has a counterpart platen roller with which a respective print head forms a respective nip and through which the receiver passes in combination with a respective color ribbon of dye. In lieu of separate platen rollers there may be a single large roller which forms a nip with each of the print heads. The mounting assemblies allow the print heads' positions to be adjusted so that the mounting assemblies can be pivoted towards and away from the respective platen rollers. In this regard, the mounting assemblies are pivotable between an “up” position wherein the print heads are disengaged from the platen rollers and a “down” position wherein the print heads are in biased engagement with the platen rollers.
A problem with printer apparatus of the type described above is the difficulty of properly aligning the color separations on the receiver to give crisp, high quality images. Even when the print heads are accurately positioned and relative to the print drum or to the receiver pathway, there still exists the possibility for poor registration which deteriorates print quality. There is a possibility for misregistration in the direction of travel of the receiver because the receiver may stretch or become misaligned on the drum. U.S. Pat. No. 5,196,864, which issued to H. R. Caine on Mar. 23, 1993, addresses many causes of such poor registration.
Even when these causes of poor registration are negated, there exists a risk of improper color separation alignment due to changes in the velocity of the image receiver as a function of changes in the temperature of the receiver. It has been determined that the penetration depth of the drive features of a capstan roller of the receiver conveyance system is one cause of this change in receiver velocity.
As a printer is operated in such a manor as to produce many multiple prints without stop periods between prints, the internal components of the printer will retain thermal energy. Specifically, the temperature of print heads, their associated platen rollers, and other surfaces in the conveyance path that contact the receiver will increase. The internal air temperature will also increase. The overall change in temperature alters the transport characteristics of the image receiver. This change results in reducing the transport velocity of the receiver.